DESCRIPTION (Applicant?s Abstract) The most rapid phase of post-natal ocular growth occurs during the first years of life. This period is characterized by emmetropization, a reduction in both the average amount and the variability in refractive error. The vast majority of infants reach an emmetropic refractive state by three years of age and have no significant refractive error. Despite continued growth of the eye after the age of three, emmetropia is maintained in most children until the age of about nine years. At that time the prevalence of myopia begins to increase even though ocular growth is slower on average compared to age three. It is not understood how emmetropia is maintained during more rapid growth in early childhood, yet fails to be sustained throughout the period of slower growth in the school years. The initial phase of the Berkeley Infant Biometry Study (BIBS) was a five-year longitudinal study of the ocular components in infants and toddlers aged three months to three years. It investigated the changes occurring in the eyes of infants and toddlers during the ernmetropizing period of ocular development. The goal of the continuation of this project over the next three years is to determine how emmetropia is maintained after three years of age. We will evaluate competing hypotheses that emmetropia is maintained by visual feedback from defocus or accommodative lag as opposed to being maintained by passive processes where innate axial growth is balanced by lens power changes due to decreases in equivalent refractive index. This goal will be accomplished by longitudinal measurement of refractive error, accommodative response, corneal curvature, anterior chamber depth, crystalline lens thickness and surface curvatures, vitreous chamber depth, and axial length in the original BIBS participants when they are between 3 and 6 years of age. Besides establishing normative growth curves for all the eye?s optical components for the first six years of life, these data will allow for evaluation of predictive factors for mature refractive error based on the earlier values for these parameters. Longitudinal refractive and biometric data from infancy through early childhood will make it possible to investigate the responsiveness of the eye to defocus, what components are responsible for maintaining emmetropia, the role of accommodation in ocular growth, and how component and refractive development in the first years of life are related to those parameters in later childhood.